The present invention relates to a manufacturing method of a spectacle lens to correct eyesight.
In general, a spectacle lens Is custom-made to meet the customer's specification. However, it takes long time to process both front and back surf aces after receiving the customer's order. Therefore, semifinished lens blanks whose front surfaces are finished are stockpiled and a back surf ace of the selected semifinished lens blank is processed according to the customer's specification in order to shorten delivery times. Further, the entire range of available vertex power of a spectacle lens Is divided into about ten sections, and one type of the semifinished lens blank is prepared for each of the sections.
Aspherical spectacle lenses whose at least one of the front and back surfaces is aspherical have come into wide use. When the spectacle lens employs an aspherical surf ace, the base curve becomes slower (i.e., the absolute value of the front vertex power decreases) and the maximum thickness becomes shorter as compared with a spherical lens whose both of the front and back surfaces are spherical. A conventional semifinished lens blank prepared for an aspherical spectacle lens has an aspherical finished front surface. A back surface thereof will be processed to be spherical or toric to meet the customer's specification.
FIGS. 27A through 27C show a sample of the sections of the vertex power, FIG. 27A shows a range of minus diopter, FIG. 27B shows a range of plus diopter and FIG. 27C shows a range of mixed diopter. The entire range of the available vertex power, which is a combination of a spherical power SPH and a cylindrical power CYL, is divided into nine sections I through IX. One type of the semifinished lens blank is prepared for each of the sections. The relationship between the sections and the base curves of the semifinished lens blank is shown in TABLE 1 and FIG. 28.
TABLE 1SectionBase curve (D)I0.50II1.25III2.00IV3.00V4.00VI5.00VII6.00VIII7.00IX8.00——
Although the aspherical spectacle lens has advantages of slow base curve and thin maximum thickness, it has disadvantages of larger distortion at periphery and larger degradation of optical performance when the lens deviates from a regular position as compared with the spherical spectacle lens. In general, when the average power error and the astigmatism are well corrected, the slower the base curve is, the larger the distortion is. Namely, the optical performance in the periphery will be degraded as the base curve becomes slower to save weight. Since there Is an individual difference for a permissible distortion, one person may feel normal when he or she wears an aspherical spectacle lens having a slow base curve, while another person may feel abnormal and may not stand to wear the same aspherical spectacle lens.
In the conventional manufacturing method of the aspherical spectacle lens, the base curve is determined based on the vertex power alone without considerations of a problem of customer's taste such as weightings of optical performance and outward appearance, and a problem of customer's adaptability such as a permissible distortion. If the customer cannot be satisfied with the optical performance of the aspherical spectacle lens having the determined base curve, he or she has no other choice but to select a heavy spherical spectacle lens having a sharp base curve. Therefore, it is difficult to supply the most suitable spectacle lens for the customer with consideration of the customer's taste and adaptability.
Further, when the vertex powers of the right and left spectacle lenses belong to the different sections, the outward appearance lacks balance between the right and left lenses because of the difference between the base curves. On the other hand, when a finished lens having a predetermined vertex power belonging to the predetermined section is manufactured using a semifinished lens blank prepared for a different section in order to make the lenses the same outward appearance, the optical performance becomes significantly worse, because the each aspherical front surface of the semifinished lens blank is designed so as to keep an optical performance for the vertex power within the specific section.